Update style.css

removed 

	margin:0;
	padding:0;
	height:100%;
        background-color:#EDEDED;

from html, body {}

Added 

body {
    height:1000px;
    width:100%;
    background-color:#F0F0F0;
}

#header_nav {
    width:100%;
    height:100px;
    background-color:#666;
    position:fixed;
    top:0;
    left:0;
}